On Thu, Aug 14, 2003 at 02:40:22PM -0400, Greg A. Woods wrote:
> unsigned long inet = 0; /* an IPv4 address */
>
> printf("parse_address(): inet addr given: [%s]\n",
> inet_ntoa(*((struct in_addr *) &inet)));
This code is completely bogus and will not work on any 64 bit platform,
besides maybe by a lot of luck and/or a buggy compiler.
On my alpha:
sizeof(unsigned long) = 8, sizeof(struct in_addr) = 4
So what do you expact the dereference of the casted pointer to mean? Should
it pass a 4 byte object to inet_ntoa (and which half of the 8 byte object
you started from)?
Anyway, C99 does not allow such casts, and accordingly gcc 3.3.1 warns about
it, with the now already famous:
test.c:14: warning: dereferencing type-punned pointer will break strict-aliasing rules
While this is only a warning, the generated code does not do what you expect
it to do. Example (on sparc64, gcc 3.3.1, with inet = 0 replaced by
inet = 0x0102030405060708):
Compiled with -O (does not include -fstrict-alias):
parse_address(): inet addr given: [1.2.3.4]
Compiled with -O2:
parse_address(): inet addr given: [1.0.0.0]
This is *not* a compiler bug. According to the C standard you are invoking
undefined behaviour.
Martin